There is an optical disc apparatus which can read and/or write information from/on three types of optical discs: CD (compact disc), DVD (digital versatile disc), and BD (Blu-ray disc). A CD has a track pitch of 1.6 μm and a capacity of 650 MB, and an infrared laser light with a wavelength of about 0.78 μm and an objective lens with a numerical aperture (NA) of 0.45 are used for recording or reproducing information on/from a CD. Further, a DVD has a track pitch of 0.74 μm and a capacity of 4.7 GB, and a red laser light with a wavelength of about 0.65 μm and an objective lens with an NA of 0.6 are used for recording or reproducing information on/from a DVD. Furthermore, a BD has a track pitch of 0.32 μm and a capacity of 25 GB, and a blue laser light with a wavelength of about 0.405 μm and an objective lens with an NA of 0.85 are used for recording or reproducing information on/from a BD.
An optical disc apparatus compatible with a plurality of types of optical discs must discriminate a type of inserted optical disc and be set up in accordance with the inserted optical disc. As a conventional optical disc apparatus, there is an apparatus which adopts a method of detecting a diffraction angle of a diffraction light of a laser light beam impinging on the optical disc, by using a two-divided light-receiving element or two separate light-receiving elements, and discriminating a type of the optical disc on the basis of a result of the detection (see patent document 1, for example).
Patent document 1 is Japanese Patent Application Kokai Publication No. 10-154348 (paragraphs 0030 to 0040, FIG. 6 to FIG. 11).